Set env var to hold Keras at Keras 2 (huggingface#29598)

* Set env var to hold Keras at Keras 2

* Add Amy's update

* make fixup

* Use a warning instead

src/transformers/modeling_tf_utils.py

@@ -78,6 +78,16 @@
 if TYPE_CHECKING:
     from . import PreTrainedTokenizerBase
 
+logger = logging.get_logger(__name__)
+
+if "TF_USE_LEGACY_KERAS" not in os.environ:
+    os.environ["TF_USE_LEGACY_KERAS"] = "1"  # Compatibility fix to make sure tf.keras stays at Keras 2
+elif os.environ["TF_USE_LEGACY_KERAS"] != "1":
+    logger.warning(
+        "Transformers is only compatible with Keras 2, but you have explicitly set `TF_USE_LEGACY_KERAS` to `0`. "
+        "This may result in unexpected behaviour or errors if Keras 3 objects are passed to Transformers models."
+    )
+
 try:
     import tf_keras as keras
     from tf_keras import backend as K
@@ -93,7 +103,6 @@
         )
 
 
-logger = logging.get_logger(__name__)
 tf_logger = tf.get_logger()
 
 TFModelInputType = Union[